Measuring and neutralizing the electrical charge at the interface of a magnetic head and media

ABSTRACT

A system and method for measuring and neutralizing the electrical charge at the interface of a magnetic head and a magnetic storage medium, such as a disk, is disclosed. A surface treatment material is applied to the magnetic head. The surface treatment material matches the medium surface material on the surface of the magnetic storage medium. The surface treatment material on the magnetic read/write head may be a fluorinated carbon, such as a Fomblin Z-derivative, perfluoro alkyl trichlorosilane, a FC-722, or a fluorinated polymer. The surface treatment material can be applied to the magnetic head by a vapor deposition process or by a liquid immersion process. The charge on the head-disk interface can be measured by applying varying external charges to the head while reading a signal previously written to the disk.

BACKGROUND INFORMATION

The present invention relates to magnetic hard disk drives. Morespecifically, the present invention relates to a method of measuring andneutralizing the electrical charge at the interface of the magnetic headand the magnetic storage media.

Hard disk drives are common information storage devices essentiallyconsisting of a series of rotatable disks that are accessed by magneticreading and writing elements. These data transferring elements, commonlyknown as transducers, are typically carried by and embedded in a sliderbody that is held in a close relative position over discrete data tracksformed on a disk to permit a read or write operation to be carried out.In order to properly position the transducer with respect to the disksurface, an air bearing surface (ABS) formed on the slider bodyexperiences a fluid air flow that provides sufficient lift force to“fly” the slider and transducer above the disk data tracks. The highspeed rotation of a magnetic disk generates a stream of air flow or windalong its surface in a direction substantially parallel to thetangential velocity of the disk. The air flow cooperates with the ABS ofthe slider body which enables the slider to fly above the spinning disk.In effect, the suspended slider is physically separated from the disksurface through this self-actuating air bearing.

Some of the major objectives in ABS designs are to fly the slider andits accompanying transducer as close as possible to the surface of therotating disk, and to uniformly maintain that constant close distanceregardless of variable flying conditions. The height or separation gapbetween the air bearing slider and the spinning magnetic disk iscommonly defined as the flying height. In general, the mountedtransducer or read/write element flies only approximately a fewnanometers above the surface of the rotating disk. The flying height ofthe slider is viewed as one of the most critical parameters affectingthe magnetic disk reading and recording capabilities of a mountedread/write element. A relatively small flying height allows thetransducer to achieve greater resolution between different data bitlocations on the disk surface, thus improving data density and storagecapacity. With the increasing popularity of lightweight and compactnotebook type computers that utilize relatively small yet powerful diskdrives, the need for a progressively lower flying height has continuallygrown.

As shown in FIG. 1 an ABS design known for a common catamaran slider 5may be formed with a pair of parallel rails 2 and 4 that extend alongthe outer edges of the slider surface facing the disk. Other ABSconfigurations including three or more additional rails, with varioussurface areas and geometries, have also been developed. The two rails 2and 4 typically run along at least a portion of the slider body lengthfrom the leading edge 6 to the trailing edge 8. The leading edge 6 isdefined as the edge of the slider that the rotating disk passes beforerunning the length of the slider 5 towards a trailing edge 8. As shown,the leading edge 6 may be tapered despite the large undesirabletolerance typically associated with this machining process. Thetransducer or magnetic element 7 is typically mounted at some locationalong the trailing edge 8 of the slider as shown in FIG. 1. The rails 2and 4 form an air bearing surface on which the slider flies, and providethe necessary lift upon contact with the air flow created by thespinning disk. As the disk rotates, the generated wind or air flow runsalong underneath, and in between, the catamaran slider rails 2 and 4. Asthe air flow passes beneath the rails 2 and 4, the air pressure betweenthe rails and the disk increases thereby providing positivepressurization and lift. Catamaran sliders generally create a sufficientamount of lift, or positive load force, to cause the slider to fly atappropriate heights above the rotating disk. In the absence of the rails2 and 4, the large surface area of the slider body 5 would produce anexcessively large air bearing surface area. In general, as the airbearing surface area increases, the amount of lift created is alsoincreased. Without rails, the slider would therefore fly too far fromthe rotating disk thereby foregoing all of the described benefits ofhaving a low flying height.

As illustrated in FIG. 2, a head gimbal assembly 40 often provides theslider with multiple degrees of freedom such as vertical spacing, orpitch angle and roll angle which describe the flying height of theslider. As shown in FIG. 2, a suspension 74 holds the HGA 40 over themoving disk 76 (having edge 70) and moving in the direction indicated byarrow 80. In operation of the disk drive shown in FIG. 2, an actuator 72(such as a voice-coil motor (VCM)) moves the HGA over various diametersof the disk 76 (e.g., inner diameter (ID), middle diameter (MD) andouter diameter (OD)) over arc 75.

The electrical charge at the magnetic head and disk interface can causeserious tribology and reliability problems, such as lubricationdegradation, head-disk spacing change, and electrostatic damage of themagnetic sensor. With spacing between the head and disk getting smallerfor higher recording density, there is an increased chance of head-diskinteractions and therefore, more electrical charge at the head-diskinterface due to tribo-charge effects.

In view of the above, there is a need for a system and a method ofmeasuring and reducing accumulated charge on the magnetic read/writehead.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a slider device with a read/write headthat is known in the art.

FIG. 2 is a perspective view of a disk drive device that is known in theart.

FIG. 3 provides an illustration of a head disk interface (HDI) chargemeasurement apparatus according to an embodiment of the presentinvention.

FIG. 4 provides a flowchart illustrating an HDI charge measurementprocess according to an embodiment of the present invention.

FIG. 5 provides a graph illustrating an HDI charge measurement readingaccording to an embodiment of the present invention.

FIG. 6 provides a graph comparing an HDI charge measurement reading of acoated magnetic head and a non-coated magnetic head according to anembodiment of the present invention.

FIG. 7 provides an illustration of an HDI charge preventative coatingapplication according to an embodiment of the present invention.

FIG. 8 provides a flowchart illustrating a vapor deposition process forapplying an HDI charge preventative coating according to an embodimentof the present invention.

FIG. 9 provides an illustration of a liquid immersion system forapplying an HDI charge preventative coating according to an embodimentof the present invention.

FIG. 10 provides a flowchart illustrating a liquid immersion process forapplying an HDI charge preventative coating according to an embodimentof the present invention.

FIG. 11 provides a graph illustrating an HDI charge measurement readingof an alternative HDI charge preventative coating according to anembodiment of the present invention.

DETAILED DESCRIPTION

A system and method for measuring and neutralizing an electrical chargeat the interface of a magnetic head and a magnetic storage medium, suchas a disk, is disclosed. In one embodiment, the charge on the head-diskinterface may be measured by applying varying external charges to thehead while reading a signal previously written to the disk. In a furtherembodiment, a surface treatment material is applied to the magnetic headto neutralize electrical charge on the magnetic head. The surfacetreatment material may match the surface material of the magneticstorage medium. In a further embodiment, both materials may be afluorocarbon, such as a Fomblin Z-derivative, fluorinatedalkyl-trichlorosilane, fluorinated alkyl-trialkyloxysilane, andfluorinated surfactants. In one embodiment, the surface treatmentmaterial may be applied to the magnetic head by a vapor depositionprocess or by a liquid immersion process.

According to an embodiment of the present invention, a process fordetermining the head-disk interface (HDI) charge uses variations inreadings of a stored signal caused by the application of an externalvoltage to the magnetic head. FIG. 3 illustrates one embodiment of a HDIcharge measurement apparatus. In one embodiment, a magnetic head 300reads data from and writes data to a magnetic data storage medium, suchas a disk 310. In a further embodiment, the magnetic head 300 sends asignal to a pre-amplifier 320, which amplifies the signal before sendingthe signal on to a processing unit 330 for processing the data. In anadditional embodiment, the disk 310 rotates on a spindle 340 that iscoupled to ground 350. In one embodiment, the magnetic head 300 isisolated from the ground 350. The magnetic head 300 is also coupled to avariable charge or voltage source 360 via an isolated suspensionmounting 370.

FIG. 4 illustrates in a flowchart one embodiment of an HDI chargemeasurement process. The process starts (Block 400) by engaging themagnetic head 300 relative to the disk 310 (Block 410). The magnetichead 300 writes a signal to one or more tracks on the disk 310 (Block420). Once written to the surface, this signal can also serve as anindicator of head-disk spacing during the write operation. The magnetichead 300 then reads the signal back from the disk 310 at zero externalvoltage from supply 360 (Block 430). The magnetic head 300 then readsthe written signals on the disk 310 with different external voltagesapplied by the voltage source 360 (Block 440). In one embodiment, theexternal voltage can be varied from −5 to 5 volts, depending on theactual spacing between the head 300 and the disk 310. After a set numberof external voltages have been used, the process is finished (Block450).

FIG. 5 illustrates a graph of an HDI charge measurement resulting fromthe HDI charge measurement process according to an embodiment of thepresent invention. In the graph, the pulse width at 50% (PW50) inmicro-inches is plotted against voltage in volts. The pulse width at 50%is found by measuring the distance between when the pulse is at 50% ofthe amplitude on the ascending slope and when the pulse is at 50% of theamplitude on the descending slope. In an embodiment in which no HDIcharge is present, the PW50 should decrease when either positive ornegative voltage is applied to the magnetic head 300, due to reducedspacing at the head-disk interface. In one embodiment, the relationshipbetween the head-disk interface and electric charge can be modeled as aquasi-parallel capacitor, where the head-disk spacing (d), voltageapplied (V), and the attractive force (f) relationship follows theequation f=k V²/d², where k is a constant. As a result of thisrelationship, the spacing d decreases with increasing V after the forcesacting upon the head are at equilibrium. As is seen by the graph in thisembodiment, the PW50 increases as a negative voltage is applied untilreaching a maximum 500, at which point the PW50 begins to decrease. Themaximum 500 is offset from the zero voltage position due to a negativecharge being present on the disk surface. This negative charge willrepel the same charge from the opposite surface of the magnetic head orcancel the effects of charging from the opposite surface, leading to anincreased PW50. When this negative charge is overcome by the externalcharge/voltage, then the PW50 decreases again with the applied voltage.In one embodiment, the HDI charge is given as the voltage thatcorresponds to the peak PW50 value. In an alternate embodiment, the HDIcharge is given as PW50 delta for a given absolute voltage. One way todepict the asymmetry of PW50 versus voltage curve is to arbitrarilydefine the delta as the difference between PW50 at +2V and PW50 at −2V.The bigger the PW50 delta, the more asymmetry of the PW50 versus voltagecurves.

In one embodiment, the HDI charge is caused by the lubricants present onthe disk 310 surface to protect the disk surface against stiction andmechanical wear. Often the lubricant used is a fluorinated carbon, suchas a Fomblin Z derivative. Fluorinated carbons have a high affinity forelectrons, because of the presence of fluorine. When the disk surface isrubbed, either during the disk fabrication process or during takeoff ofthe magnetic head, the tribo-charge process generates electrons andpositive ions. Due to its high affinity for electrons, a fluorinatedpolymer will attract electrons and then assumes a negativecharge/potential. Furthermore, since the fluorinated polymer is adielectric material, the collected negative charge will not quicklydissipate through the disk substrate. Instead, the negative charge isgradually neutralized through the absorption of molecules with positivecharges from the environment. However, the head flying over the diskproduces a sub-ambient pressure, causing the absorbed molecules to bequickly evaporated and brings back the negative charge effect. Thenegative charge on the disk surface induces a positive charge on themagnetic head, forming a quasi-parallel capacitor with an attractionforce between the head and the disk. In one embodiment, this iscounteracted by coating the magnetic head with a thin layer of materialthat is the same as or similar to the coating on the disk, causing bothsurfaces to exhibit the same sign of electrical charge when they arebrought together in the HDI of a disk drive. Because of the same orsimilar surface charge, the attractive forces between the head and diskshould be minimized or eliminated. FIG. 6 illustrates in a graph acomparison of an embodiment of a magnetic head with the coating 600 andan embodiment of a magnetic head without the coating 610. The magnetichead with the coating 600 has a maximum PW50 measurement at zeroexternal voltage. The magnetic head without the coating 610 has amaximum at an external voltage of around −0.5 volts.

In one embodiment, a fluorinated carbon, such as perfluoro decyltrichlorosilane (PFDTS), may be applied either by vapor depositionprocess or liquid immersion process. In another embodiment of thepresent invention, using the system of FIG. 7, a vacuum coating processfor applying the monolayer surface coating to the magnetic head isprovided and illustrated in the flowchart of FIG. 8. The process starts(Block 800) by filling monolayer agent 710 into a glass flask 720 (Block810), which then provides low pressure vapor 730 of the active agent tothe coating chamber 740. In this example, the monolayer agent 710 ispure (96% or greater) PFDTS in the glass flask 720, which is heated upby heating tape 750 to 100° C. to obtain higher vapor pressure. Atemperature controller 760 may control the heating of the monolayeragent 710. The magnetic read/write head 300 is hung by a rack 770 in thecoating chamber (Block 820). The coating chamber 740 is cleaned first bypumping down to a low vacuum level and backfilling with nitrogen gas fora few cycles to remove residual moisture (Block 830). The magneticread/write head 300 is then exposed to the vapor 730 of the monolayeragent for 30 minutes (Block 840). After the valve 780 for active agentis closed, the coating chamber 740 is cleaned again by pumping down tolow vacuum and backfilling with nitrogen gas for a few cycles to removeexcess coating and byproducts (Block 850). The magnetic read/write head300 is removed from the chamber 740 (Block 860), ending the process(Block 870). The length of time varies depending on requirements forchamber cleanliness and coating quality. In one embodiment, the chambertemperature was 105 degrees Celsius. However, monolayer coating has beensuccessfully deposited on the substrate in a wide range of temperaturesbetween 20° C. to 250° C.

In an alternate embodiment, FC-722, a fluorinated carbon produced by3M®, is used as the surface coating, the FC-722 having similar electronaffinities as the surface coating of the magnetic media storage. In oneembodiment, FC-722 is deposited on the head using a liquid immersionprocess. In one embodiment of the present invention, using the system ofFIG. 9, a liquid process is provided for applying the monolayer surfacecoating to the magnetic head and is illustrated in the flowchart of FIG.10. The process starts (Block 1000) by filling a container 910 filledwith a solution 920 (Block 1010). The solution may be a mixture offluorinated solvent, such as PF5060 by 3M®, and the active agent FC-722.The magnetic read/write heads 300 may be hung from a rack 930 andimmersed into the monolayer solution 920 (Block 1020). After 10 minutes,the magnetic read/write head 300 is removed from the monolayer solution920 by draining the monolayer solution 930 out through a bottom drain940 (Block 1030). The magnetic heads may be removed and placed in anoven at 120 degrees Celsius for 30 minutes to remove excess solvent andto cure the coating (Block 1040), ending the process (Block 1050).

FIG. 11 illustrates in a graph one embodiment of an HDI chargemeasurement of a first head 1100 and a second head 1110, each havingreceived a surface coating of FC-722.

Although several embodiments are specifically illustrated and describedherein, it will be appreciated that modifications and variations of thepresent invention are covered by the above teachings and within thepurview of the appended claims without departing from the spirit andintended scope of the invention.

1-19. (canceled)
 20. A method, comprising: coating a magnetic storagemedium with a medium surface material; and coating a magnetic head toread data from and write data to the magnetic storage medium with a headsurface treatment material matching the medium surface material.
 21. Themethod of claim 20, wherein the surface treatment material is afluorinated polymer.
 22. The method of claim 20, further includingapplying the surface treatment material using a vapor depositionprocess.
 23. The method of claim 22, wherein the vapor depositionprocess includes: suspending the magnetic head above fluorinated carbonliquid in a nitrogen purged box at room temperature; moving the magnetichead to a vacuum oven; and baking the magnetic head for one hour at 110degrees Celsius.
 24. The method of claim 20, further including applyingthe surface treatment material using a liquid immersion process.
 25. Themethod of claim 24, wherein the liquid immersion process includes:dissolving fluorinated carbon agents into a pool of solvents, creating afluorinated carbon liquid; immersing the magnetic head in thefluorinated carbon liquid; removing the magnetic head from thefluorinated carbon liquid; and cleaning excess coating materials fromthe magnetic head.
 26. The method of claim 20, wherein the magneticstorage medium includes a magnetic disk coupled to a ground via aspindle.
 27. The method of claim 26, further including electricallyisolating the magnetic head from the ground.
 28. The method of claim 27,further including applying an external electrical charge to the magnetichead.
 29. The method of claim 28, further including taking a measurementof an interface charge.
 30. The method of claim 29, wherein taking ameasurement includes: writing a signal on the magnetic storage medium;reading the signal back with zero external charge applied to themagnetic head; and reading the signal back with varying externalcharges.